Physical vapour deposition by means of sputtering has become a standard technique to customise the properties of for example glass panes or other rigid or flexible materials. ‘Sputtering’ refers to the ballistic ejection of coating material atoms out of a target by means of positively charged ions, —usually argon—that are accelerated by an electric field towards a negatively charged target. The positive ions are formed by impact ionisation in the low pressure gas phase. The ejected atoms impinge on the substrate to be coated where they form a dense, well adhering coating.
The ionisation of the gas forming the ions is confined close to the surface of the target by means of a magnetic field generated from behind the target surface and exhibiting an arc shaped, closed loop tunnel at the surface of the target. During operation, electrons bounce back and forth along those magnetic field lines while drifting down the closed loop thereby increasing the impact ionisation probability of the gas atoms. A plasma glowing closed loop ‘race track’ forms at the surface of the target.
It remains an engineering challenge when one wants to use cylindrical, rotating targets instead of the easier to implement planar stationary targets. When using the latter, the coolant supply (the target must be cooled as the impact of the positive ions heats up the target) and electrical energy supply can be done to a fixed target assembly. When using rotating targets the coolant and electric supply must be rotational compliant while maintaining vacuum integrity. However, the benefits of succeeding in this challenge are worth the effort as rotatable targets carry much more usable target material stock than planar targets do. Also rotatable targets are less prone to arcing compared to their planar counterparts. These advantages are particularly appreciated in inline coaters wherein substrates pass by the elongated, cylindrical target in a direction perpendicular to the axis of the target. In order to maintain even coating thicknesses across the substrate a uniform sputter rate of target material is needed over the length of the target.
One of the engineering problems one is faced with is that the magnetic field generator must be contained in the target. The magnetic field generator—oriented towards the substrate to be coated—is usually held stationary while the cylindrical target rotates in front of it. High performance permanent magnets based on iron neodymium boron (Fe—Nd—B) or cobalt samarium (Co—Sm) alloys are used to generate the magnetic field. As the component of the magnetic field parallel to the surface of the target is determining the confinement of the electrons in the plasma it is important that this component is controlled along the length of the tube. Unfortunately, the magnetic induction (in tesla) of this component normally drops with at least the second power of the distance to its generator and hence is very sensitive to the position of the magnetic field generator with respect to the target surface. The distance between the target surface and the magnetic field generator must therefore be well controlled as otherwise the plasma would show local variations in intensity that in their turn can lead to non-uniform coating profiles across the substrate.
WO 2009/138348 discloses a solution to the problem of controlling the distance between the target surface and the magnetic field generator: an adjustable mounting of the magnetic field generator makes it possible to adjust the distance between the magnetic field generator and the tube outer wall. In this publication, adjustment of the distance of the magnetic field generator with respect to the outer wall of the tube is enabled by discrete supports distributed over the length of the tube, which discrete supports may be made longer or shorter depending on the needs. Mechanical systems may be used thereto, such as for example introduction of shims or washers over a screw that holds the support in place to increase the overall length of the support, or introduction of an adjustment screw in the support where the screw turns in a threading fixedly connected to the tube while the end of the screw is axially held by a holder connected to the generator in which it can freely turn.
It is a disadvantage of the teaching of the above publication that the magnetron has to be opened, hence the vacuum has to be removed, in order to allow for adjustments, and after the adjustment has been carried out, the vacuum has to be re-applied. This is very time consuming.